There are many anecdotal guidelines to when one should become concerned about a crack. The width of the crack is only one aspect of crack interpretation and even though you might not be able to put your hand through it, if you see daylight on the other side then its certainly time to become concerned.
The cracking on this blog is as a result of an extension built on a raft foundation in Galway City about ten years ago. The extension is rotating away from the original dwelling. It is in fact ‘tilt’ where the foundation or building moves as one unit with little or no cracking or distortion on the extension itself. Factors that influence the ability of a structure to tilt are the geometry of the building and the reinforcement of the raft. And of course, the ground conditions.
Tilt is normally expressed as a fraction and research by the Building Research Establishment have provided guidance on indicative values from where a building becomes noticeable (1/250) to where monitoring or remedial action (1/100) is required.
In this instance, a trial hole revealed that the raft was built on fill which was poorly selected and placed. Unfortunately, there was no supervising engineer employed for the works and the builder was allowed to design and construct the foundation himself.